Circuits and methods for slew rate control of switched capacitor regulators

ABSTRACT

Circuits comprising: a first capacitor(C1); a first switch(S1) having a first side coupled to a VIN and a second side coupled to a first side of C1; a second switch(S2) having a first side coupled to the second side of S1; a third switch(S3) having a first side coupled to a second side of S2 and a second side coupled to a second side of C1; a fourth switch(S4) having a first side coupled to a second side of S3 and a second side coupled to a VSUPPLY, wherein: in a first state, S1 and S3 are off, and S2 and S4 are on; in a second state, S1 and S3 are on, and S2 and S4 are off; and at least one of a control of S1, a control of S2, a control of S3, and a control of S4 is coupled to a time-varying-slew-rate signal.

BACKGROUND

Switched capacitor regulators are a well-known class of regulator that can be used to regulate voltage or current. In many existing switched capacitor regulators, a network of switches in the regulators switch the regulators between two states. The output voltage or the output current of such regulators can be regulated by adjusting the frequency at which the regulators switch between states. However, changing the switching frequency can be prohibitive in electronic devices that are sensitive to electromagnetic interference (EMI). For example, mobile phones have strict specifications on EMI because too much EMI can affect call quality and wireless data transfer, and mobile phone engineers often need to adjust the operating frequency of various chips to prevent them from interfering with the key communication signals to ensure call quality does not degrade. To meet the strict EMI specification, the SC regulator might need to operate at a predictable, single switching frequency. In this case, the SC regulator cannot adjust the switching frequency to regulate the output voltage.

Accordingly, new mechanisms for regulating the output of switched capacitor regulators are desirable.

SUMMARY

In accordance with some embodiments, circuits and methods for slew rate control of switched capacitor regulators are provided. More particularly, in some embodiments, circuits for a switched capacitor regulator are provided, the circuits comprising: a first capacitor having a first side and a second side; a first switch having a first side coupled to an input voltage, a second side coupled to the first side of the first capacitor, and a control; a second switch having a first side coupled to the second side of the first switch, a second side, and a control; a third switch having a first side coupled to the second side of the second switch, a second side coupled to the second side of the first capacitor, and a control; a fourth switch having a first side coupled to the second side of the third switch, a second side coupled to a supply voltage, and a control, wherein in a first state: the first switch is off; the second switch is on, the third switch is off, and the fourth switch is on, wherein in a second state: the first switch is on, the second switch is off, the third switch is on, and the fourth switch is off, and wherein at least one of the control of the first switch, the control of the second switch, the control of the third switch, and the control of the fourth switch is coupled to a control signal having a slew rate that varies over time.

Still more particularly, in some of these embodiments, the first switch is a PMOS FET, and the control of the first switch is a gate of the PMOS FET.

Still more particularly, in some of these embodiments, the second switch is an NMOS FET, and the control of the second switch is a gate of the NMOS FET.

Still more particularly, in some of these embodiments, the third switch is a PMOS FET, and the control of the third switch is a gate of the PMOS FET.

Still more particularly, in some of these embodiments, the fourth switch is an NMOS FET, and the control of the fourth switch is a gate of the NMOS FET.

Still more particularly, in some of these embodiments, the supply voltage is ground.

Still more particularly, in some of these embodiments, the circuits further comprise a second capacitor having a first side coupled to the second side of the second capacitor and a second side coupled to the supply voltage.

Still more particularly, in some of these embodiments, the circuits further comprise a variable capacitance having a first side coupled to one of the control of the first switch, the control of the second switch, the control of the third switch, and the control of the fourth switch. Even still more particularly, in some of these embodiments, the variable capacitance is a bank of switched capacitors. Even still more particularly, in some of these embodiments, the variable capacitance is a varactor.

Still more particularly, in some of these embodiments, the circuits further comprise a variable current source having an output; third capacitor having a first side coupled to the supply voltage and having a second side coupled to the output of the variable current source and one of the control of the first switch, the control of the second switch, the control of the third switch, and the control of the fourth switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a model of an N:M switched capacitor (SC) regulator in accordance with some embodiments.

FIG. 2A is an example of a 2:1 SC regulator in accordance with some embodiments.

FIG. 2B illustrates examples of waveforms for four gates of the 2:1 SC regulator of FIG. 2A in accordance with some embodiments.

FIGS. 3A-3C illustrate examples of gate drive waveforms with different slew rates in accordance with some embodiments.

FIG. 3D illustrates an example of a way to adjust the effective VGS by adjusting the supply voltage of the drivers that generate the gate driver signals in accordance with some embodiments.

FIG. 4A is an example of first circuit for changing slew rate in accordance with some embodiments.

FIG. 4B is an example of a second circuit for changing slew rate in accordance with some embodiments.

DETAILED DESCRIPTION

In accordance with some embodiments, mechanisms for regulating the output of switched capacitor regulators using variable slew rates on the control signals of power switches are provided.

FIG. 1 illustrates an example 100 of a model of an N:M switched capacitor (SC) regulator in accordance with some embodiments. For example, if N is 2 and M is 1, the SC regulator model 100 describes a 2:1 SC regulator. The input voltage VIN 102 is multiplied by the ratio M/N 108, which is followed by the output resistor R_(OUT) 106. V_(OUT)=M×V_(IN)/N−R_(OUT)×I_(OUT). R_(OUT) 106 can be adjusted by changing the switching frequency of the SC regulator, but that can be prohibitive due to EMI reasons in certain applications. R_(OUT) 106 can also be adjusted by changing the effective resistance of power switches. One way to do this is to change the slew rate of the gate driver signals going to the power switches.

FIG. 2A illustrates an example of a 2:1 SC regulator 102 in accordance with some embodiments. As shown, regulator 102 includes four power switches P_(TOP) 202, N_(MID) 204, P_(MID) 206, and N_(BOT) 208. Any suitable components can be used to implement these switches in some embodiments. For example, in some embodiments, P_(TOP) 202 and P_(MID) 206 can be PMOS FET power switches, while N_(MID) 204 and N_(BOT) 208 can be NMOS FET power switches.

During operation, these switches can turn on and off to cause the regulator to transition between State 0 and State 1 to regulate V_(OUT) 108 to be close to ½ of V_(IN) 104. These switches can be controlled by four gate signals P_(TOP_G) 210, N_(MID_G) 212, P_(MID_G) 214, and N_(BOT_G) 216 of the four power switches P_(TOP) 202, N_(MID) 204, P_(MID) 206, and N_(BOT) 208. The gate signals drive a respective gate of P_(TOP) 202, N_(MID) 204, P_(MID) 206, and N_(BOT) 208 to turn them on or off.

As shown in FIG. 2A, in state 0, P_(TOP) 202 and P_(MID) 206 are turned off (as indicated by dashed lines) and N_(MID) 204 and N_(BOT) 208 are turned on. In state 1, P_(TOP) 202 and P_(MID) 206 are turned on, while N_(MID) 204 and N_(BOT) 208 are turned off (as indicated by dashed lines). As illustrated in FIG. 2A, in state 0: a first side of C_(FLY) 114 can be coupled by N_(MID) 204 to a first side of a decoupling capacitor C_(OUT) 106 and an output that outputs V_(OUT) 108; and a second side of C_(FLY) 114 can be coupled by M_(BOT) 208 to ground. In state 1: the first side of C_(FLY) 114 can be coupled by P_(TOP) 202 to V_(IN) 104; and the second side of C_(FLY) 114 can be coupled by P_(MID) 206 to the first side of decoupling capacitor C_(OUT) 106 and the output that outputs V_(OUT) 108. In both state 0 and state 1, a second node of C_(OUT) 106 can be coupled to ground 110 in some embodiments.

FIG. 2B illustrates example waveforms for the four gates P_(TOP_G) 210, N_(MID_G) 212, P_(MID_G) 214, and N_(BOT_G) 216 of the four power switches P_(TOP) 202, N_(MID) 204, P_(MID) 206, and N_(BOT) 208, respectively, that can be used in some embodiments. V_(MID) 218 can be connected to V_(OUT) 108 or can be supplied by a separate voltage regulator that generates a voltage close to ½ of the input voltage V_(IN) 104, in some embodiments.

As illustrated in FIG. 2B, P_(TOP_G) 210 and P_(MID_G) 214 can share the same signal, and N_(MID_G) 212 and N_(BOT_G) 216 can share the same signal, in some embodiments. In some embodiments, there can be a dead time 220 between the turn off time of N_(MID_G) 212 and turn on time of P_(TOP_G) 210 to provide enough margin to guarantee that both switches are never turned on at the same time, which can lead to malfunction of the SC regulator. Any suitable duration dead time can be used in some embodiments.

Even when a power switch is on, there is a non-zero on-state resistance due to non-ideal parasitic resistance. The on-state resistance of power switches can be adjusted to adjust ROUT to regulate an SC regulator. The on-state resistance of a MOSFET is roughly inversely proportional to (V_(GS)−V_(th)) (V_(GS) is the gate to source voltage of a MOSFET, and V_(th) is the threshold voltage of a MOSFET), so changing the average value of V_(GS) during on-state of the switch is one way to change on-state resistance.

This can be done by adjusting the slew rate of gate drive signals. FIGS. 3A-3C illustrate examples of gate drive waveforms with different slew rates that can be as gate drive signals. By using different waveforms, one can control the on-state resistance. FIG. 3A has the highest effective V_(GS), or highest average value of V_(GS) during on-state, which leads to lowest on-state resistance on NBOT 208. FIG. 3B has lower effective V_(GS) and FIG. 3C has the lowest effective V_(GS). Decreasing slew rate on NBOT_G 216 can lead to lower effective V_(GS) and lower R_(OUT) in some embodiments. This can be applied to switches other than N_(BOT) 208 as well in some embodiments.

As illustrated in FIG. 3D, in some embodiments, another way to adjust the effective V_(GS) can be to adjust the supply voltage of the drivers that generate the gate driver signals (e.g., N_(BOT_G) 216). In FIG. 3D, N_(BOT_G) 216 switches between GND 110 and V_(LOW) 318, which is a variable voltage level that is lower voltage than V_(MID) 218. The drawback of this method is it requires an additional voltage regulator to generate the variable voltage V_(LOW) 318. V_(LOW) 318 can be any value between GND 110 and V_(MID) 218. The lower the V_(LOW) 318 voltage, the higher the on-state resistance of N_(BOT) 208. By adjusting the on-state resistance of N_(BOT) 208, the regulator can regulate the output voltage or output current.

In contrast, changing slew rate can be done with very simple circuits. Two examples of circuits that can be used to change slew rate in accordance with some embodiments are illustrated in FIGS. 4A-4B. As illustrated in FIG. 4A, there can be an adjustable capacitance 402, which can be an array of capacitors that are connected or disconnected, or a varactor, on the gate of N_(BOT) 208. More capacitance results in lower slew rate of N_(BOT_G) 216 and lower effective V_(GS) during on-state of N_(BOT) 208. As illustrated in FIG. 4B, another example is to have a variable current source 406 (which can be implemented in any suitable manner, such as, e.g., a MOSFET with adjustable gate voltage) providing charge to a capacitor 404 connected to the gate of N_(BOT) 208. As the current increases, the slew rate on N_(BOT_G) 216 increases. Both variable current and variable capacitance can be used in some embodiments to allow for a wide range of slew rate on N_(BOT_G) 216.

Similar circuits can be applied to other power switches including P_(TOP) 202, N_(MID) 204, and P_(MID) 206.

Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of the invention, which is limited only by the claims that follow. Features of the disclosed embodiments can be combined and rearranged in various ways. 

1. A circuit for a switched capacitor regulator, comprising: a first capacitor having a first side and a second side; a first switch having a first side coupled to an input voltage, a second side coupled to the first side of the first capacitor, and a control; a second switch having a first side coupled to the second side of the first switch, a second side, and a control; a third switch having a first side coupled to the second side of the second switch, a second side coupled to the second side of the first capacitor, and a control; a fourth switch having a first side coupled to the second side of the third switch, a second side coupled to a supply voltage, and a control, wherein in a first state: the first switch is off; the second switch is on, the third switch is off, and the fourth switch is on, wherein in a second state: the first switch is on, the second switch is off, the third switch is on, and the fourth switch is off, wherein at least one of the control of the first switch, the control of the second switch, the control of the third switch, and the control of the fourth switch is coupled to a control signal that is controlled to repeatedly transition between a low state a high state with a constant predetermined frequency, and wherein the control signal is controlled to have a slew rate during each transition between the low state to the high state that causes a switch corresponding to the at least one control to have an on-state resistance determined by the slew rate that regulates a voltage at the second side of the second switch while switching between the low state and the high state at the constant predetermined frequency.
 2. The circuit of claim 1, wherein the first switch is a PMOS FET, and the control of the first switch is a gate of the PMOS FET.
 3. The circuit of claim 1, wherein the second switch is an NMOS FET, and the control of the second switch is a gate of the NMOS FET.
 4. The circuit of claim 1, wherein the third switch is a PMOS FET, and the control of the third switch is a gate of the PMOS FET.
 5. The circuit of claim 1, wherein the fourth switch is an NMOS FET, and the control of the fourth switch is a gate of the NMOS FET.
 6. The circuit of claim 1, wherein the supply voltage is ground.
 7. The circuit of claim 1, further comprising a second capacitor having a first side coupled to the second side of the second capacitor switch and a second side coupled to the supply voltage.
 8. The circuit of claim 1, further comprising a variable capacitance having a first side coupled to one of the control of the first switch, the control of the second switch, the control of the third switch, and the control of the fourth switch.
 9. The circuit of claim 8, wherein the variable capacitance is a bank of switched capacitors.
 10. The circuit of claim 8, wherein the variable capacitance is a varactor.
 11. The circuit of claim 1, further comprising: a variable current source having an output; and a third capacitor having a first side coupled to the supply voltage and having a second side coupled to the output of the variable current source and one of the control of the first switch, the control of the second switch, the control of the third switch, and the control of the fourth switch. 